A Child Rescue Ireland Alert has been issued in the search for 14-year-old Svetlana Murphy.
Gardaí say they have serious concerns there is an immediate risk to her health or welfare.
Gardaí says she is in the company of 18-year-old Nojus Maculevicius, and are travelling in a Hyundai Veloster car, with registration number 132-D-13518.
Gardaí are pleading with them to “make contact with somebody and let us know that you are safe and well.”
They were sighted in Newry on Monday at around 7pm, while the car was seen in the Belfast area at around 5.35pm yesterday evening.
Svetlana was last seen at her home on Coast Road in Blackrock in Louth and is described as 5'1", of slim build, with brown hair and brown eyes.
She was wearing navy tracksuit bottoms with a red and white stripe on the leg, a black zipped top and black runners.
Nojus is 6' tall, with slim build and dark brown hair .
He was wearing dark grey Snickers work trousers and a black Snickers top when last seen.
It's believed he also may have been wearing navy tracksuit bottoms with a red and white stripe on the leg, a black zipped top and black runners.
Anyone with information is asked to contact gardaí immediately.
People are being advised not to approach the teenagers, and to instead immediately contact 999 / 112 or your local Garda station - providing as much information as possible.